The non-invasive imaging of acute cell death, including apoptosis and necrosis, has important implications in the assessment of degenerative diseases and in the monitoring of tumor treatments. The need to achieve better and earlier imaging necessitates continuous efforts to discover uptake mechanisms and develop molecular probes that provide improved binding, pharmacokinetic and biodistribution properties.
Technetium-99m (99mTc) is a gamma ray emitting isotope used in radioactive isotope medical tests, e.g., as a radioactive tracer that medical equipment can detect in the body. 99mTc is well suited to the role because it emits readily detectable 140 keV gamma rays. The half-life of 99mTc for gamma emission is 6.01 hours, which means that about fifteen sixteenths (93.7%) of it decays to 99mTc in 24 hours. The short half life of the isotope allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low.
99mTc decays to technetium-99 (Tc-99, a less excited state of the same isotope) by rearrangement of nucleons in its nucleus. Technetium-99 emits soft beta rays but no gamma rays.
Due to its short half-life, for nuclear medicine purposes, 99mTc is usually extracted from technetium-99m generators which contain Mo-99, which is the usual parent nuclide for this isotope. The majority of Mo-99 produced for Tc-99m medical use comes from fission of HEU from only four reactors around the world: NRU, Canada; BR2, Belgium; SAFARI-1, South Africa; and HFR, the Netherlands. Production from LEU is possible, and is proposed at the new OPAL reactor, Australia, as well as other sites. Activation of Mo-98 is another, currently smaller, route of production. (CRP on Production of Mo-99 from LEU or Neutron Activation, IAEA)
99mTc is used in 20 million diagnostic nuclear medical procedures every year. Approximately 85 percent of diagnostic imaging procedures in nuclear medicine use this isotope. Depending on the type of nuclear medicine procedure, the Tc-99m is tagged (or bound to) a pharmaceutical that transports the Tc-99m to its required location. For example, when Tc-99m is chemically bound to Exametazirne, the drug is able to be cross the blood brain barrier and flow through the vessels in the brain to see cerebral blood flow (it is also used for labeling white blood cells to visualize sites of infection). Tc-99m Sestamibi is used for myocardial perfusion imaging, which shows how well the blood flows through the heart. Measurements of renal function and imaging is undertaken by tagged to Mercapto Acetyl Tri Glycine, known as a MAG3 scan.
99mTc is made from the synthetic substance molybdenum-99 which is a by-product of nuclear fission. It is because of its parent nuclide, that 99mTc is so suitable to modern medicine. Molybdenum-99 has a half-life of approximately 66 hours, and decays to 99mTc, a negative beta, and an antineutrino:99Mo(Negative Beta Decay)→99mTc+β−+νwhere β−=a negative beta particle (electron), and ν=an antineutrino;99Mo(Negative Beta Decay)→99mTc+β−+νwhere β−=a negative beta particle (electron) and ν=an antineutrino, it will then undergo an isomeric transition to yield 99mTc and a monoenergetic gamma emission:99mTc→99Tc+γ.
Single photon emission computed tomography known as SPECT is a nuclear medicine imaging technique using gamma rays. In the use of technetium-99m, the radioisotope is administered to the patient and the escaping gamma rays are incident upon a gamma camera which computes and calculates the image. To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3-6 degrees. In most cases, a full 360 degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15-20 seconds is typical. This gives a total scan time of 15-20 minutes.